Magnetic bubble memories are well known in the art. The most familiar type of bubble memory is one in which a pattern of magnetic elements is formed adjacent the surface of a film in which the bubbles are moved. The elements are designed such that repetitive patterns of magnetic poles move along channels defined by the elements in response to a magnetic drive field reorienting in the plane of bubble movement. The elements, generally, are composed of magnetically soft permalloy and have T and bar shapes in the most familiar prior art circuits.
A magnetic bubble appears as a disk when observed under a microscope through an analyzer in the presence of polarized light as is well known. The diameter of the disk in a given material is determined by a bias field antiparallel to the magnetization of the bubble. Typically, an epitaxially grown garnet film defines the plane of bubble movement and the magnetization of the film is normal to the plane. Thus, the bias field also is oriented normal to the plane of bubble movement in a direction antiparallel to the magnetization of the bubble. The reorienting (typically rotating) magnetic in-plane field can be understood to generate pole patterns in the permalloy elements. These pole patterns, in turn, modify the bias field locally causing similar patterns of magnetic field gradients which result in bubble movement. Due to the fact that bubble movement is caused by a magnetic field rather than by an arrangement of electrical conductors, and due to the fact that the permalloy elements are operative to move those bubbles to a detector for accessing sequentially information represented by the bubble pattern, this form of bubble memory is commonly called a "field-access" (or field-accessed) bubble memory.
A field-access bubble memory is commonly organized in a "major-minor" configuration. The term major-minor describes an organization for the pattern of permalloy elements which defines a plurality of closed loop paths, or minor loops, about which bubble patterns move in parallel, and an accessing path called the major path. The plurality of minor loops is operative as a permanent store. Information in the form of bubble patterns is moved from the minor loops to the major path at a reference position (or stage) defined at a turn element in each minor loop. To this end, an interchannel bubble movement-controlling permalloy pattern is formed between the turn element in each loop and an associated stage of the major path. An electrical conductor couples the turn elements of the various loops, electrically in series, causing the information, represented by the bubble pattern, to be moved in parallel to the major path when the conductor is pulsed.
The information (viz: bubble pattern) can be moved between channels in a variety of modes. In one of these modes, bubble transfer occurs. In this mode, a permalloy pattern defines a bubble transfer function operative in response to a signal in a cooperating transfer conductor to move information from the reference positions in the minor loops to the major path leaving vacancies at the reference positions. In this type of bubble memory, the major path is in the form of a closed loop also and the number of stages in the major loop is related to the number in each minor loop such that as the in-plane field continues to rotate, the transferred information and the vacancies from which the information originated move about the various paths and arrive at the transfer positions simultaneously. A second signal on the transfer conductor is operative to restore the transferred information to the originating positions.
An alternative mode of operation for bubble memories results when the permalloy pattern defines a plurality of bubble replicators. A bubble replicator is composed of a permalloy pattern at which an image of the bubble pattern (in the reference positions in the minor loops for example) is produced in the accessing path. Again, an electrical conductor couples the permalloy pattern electrically in series for controlling the replication. This form of bubble memory has the virtue that image information need not be returned to the originating vacancies, thus leading to faster cycle times.
In still another mode of operation, the permalloy pattern defines a swap function. In this mode, newly written information is exchanged at reference positions with selected information previously stored in the minor loops. Once again the operation is controlled typically by a pulsed conductor over which a permalloy pattern is formed. The transfer, swap, and replicate functions are just examples of the many special functions employed in the operation of a field-access bubble memory. Other examples are generation, annihilation, etc.
Each one of these functions presently requires the presence of a conductor pattern of one configuration and a permalloy pattern of another configuration. Where the two patterns differ (that is where they cross one another as is the usual case), a step coverage problem may exist. Because the permalloy pattern is typically formed last, the step coverage problem results in steps in the permalloy and in a permalloy pattern which is formed in more than one plane.
These steps can lead to restraints in the movement of bubbles in the layer of bubble material. The restraints have been found to be due to the effect of the in-plane field on the permalloy between two steps where a thinning of the permalloy commonly occurs. The permalloy pattern itself is in a plane normal to the direction of the bias field and parallel to that of the in-plane field. Thus the former has no effect on the permalloy pattern and the latter normally produces poles at the extremes of each element in a flux path therein aligned with the field. But, where a step occurs at an element, permalloy at the step is relatively thin. This thinning of the permalloy may give rise to pole configurations with significant separation (flux paths) between steps for providing a long path to one side of the step and a short path to the other. A net disruptive pole configuration frequently occurs and constitutes an impediment to bubble movement.